Chris Cullivertore his ACL on Thursday, a source who spoke to the cornerback told NFL.com's Ian Rapoport on Thursday night. An MRI confirmed the injury, which occurred during a punt return drill in practice.

It's exceedingly likely that Culliver will undergo surgery that will wipe out his 2013 season.

The Sacramento Bee reported earlier Thursday that the cornerback was sprinting downfield in a one-on-one punt coverage drill with rookie Marcus Cooper when Culliver crumpled to the ground. Grasping his left knee, Culliver was carted to the 49ers' training room.

Eric Branch of the San Francisco Chronicle tweeted that Culliver -- being comforted by general manager Trent Baalke -- "was in tears" as he left the practice field.

Culliver is not a starter for Jim Harbaugh's defense, but he was an important piece of the secondary who saw plenty of work in passing situations last season.

With Culliver gone, it clears what had been a logjam in the Niners' secondary. Both Nnamdi Asomugha and Carlos Rogers now are much better bets to stick with the team.